put in 8 3/4, now have vibration

Depends on the bearing surface of the slip, if that slip is 9" long than there should be plenty in the bushing.
detroit made slips for mopar ranging from 5" to 9.8" long, some were counter bored 1" some were countered 3", there are so many different factory slips for the only 2 transmissions mopar used that it is amazing

It's not the spline engagement, what people don't understand is the spline engagement only needs to be the same as the diameter of the spline...

If the spline is for example that of the 727 1.3, then the spline engagement only needs to be 1.3".... It's what is in the bushing, the trans tail bushings are anywhere from 1.37 to 2.8 in trans tailhousings and it's how much of the ground hub is in that, if you have 2.5 or so in then the slip should be supported.

You need to give more detail on the vibe, does it come in at ___ speed and it gets better or worse as speed increases, does it ever get better after a certain speed....

You changed several things at once, i had thought you only dropped one center for another and went from most normal fact ratios of 2.9 or 3.2 to 3.9 and that alone can make a vibe as now the shaft is at a rpm it never was before..

Now why the shaft was shortened is beyond me, if you couldn't get the shaft in then you had a reason to shorten it, otherwise the instruction you were given to measure from where to where when you have the shaft is also beyond me why that was done.

I do driveline work, and yes many people get online and ask others what to do and get a measurment form others and then they got a problem, and then they gotta pay twice. It's why i always say, measure your own car and ask what your local driveline shop wants you to do.

I can't tell you how many people shortened a shaft and then had to spend more for a longer slip or retube, or in many cases make a new one..

I can also tell you it can also be in the shops balancing, but i need more detail of whats going on.
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